A wireless mesh network (WMN) 100 is formed by multiple, possibly hundreds or thousands or even more of radio nodes 102, 104 that may communicate with each other, depending on e.g. transmission range, frequency channel usage, etc. The wireless mesh network 100 may have one or more sink nodes 106 that may be part of gateways to other networks, e.g. Internet 108.
FIG. 1 presents an example of the wireless mesh network 100 that may not be in static radio environment and part of the nodes may move, appear, or disappear. Therefore, the example mesh network 100 is self-organizing, and every node 102,104 may do decisions independently, but supporting the network 100 and its data delivery functionality.
One example of the wireless mesh network 100 is a wireless sensor network (WSN) formed by sensor devices 102, 104 that produce data. Each sensor device 102, 104 may be equipped with one or more radio that is used to deliver the data towards the sink node 106. Even if a single sensor radio cannot directly reach the sink node 106, the wireless mesh network 100 formed between the sensor radio nodes 102, 104 takes care of it. A routing protocol implemented in each radio node 102, 104 chooses the way to the sink node 106. Similarly, there may be data that is delivered, over multiple radio hops, from the sink node 106 to the node(s) 102, 104 or in between nodes 102, 104.
Typically, the wireless mesh network 100 may be formed by nodes 102, 104 that have limited energy resources, like batteries, while other nodes 102, 104 may be mains powered. The battery-operated nodes 102, 104 need to restrict their operation in very short time periods to not to drain the batteries more than necessary. In the wireless mesh network 100 this means that the radio nodes 102, 104 need to be synchronized to each other. Using the synchronization and a Medium Access Control (MAC) communication protocol the nodes 102, 104 know when they should switch their transceiver on.
In addition, the transmission power of the battery-operated nodes 102, 104 should be optimized so that they only use transmitter power sufficient to reach the targeted neighbour node 102, 104. This not only saves the battery of the node 102, 104, but also to limit the radio interference in the wireless mesh network 100. The mains powered nodes 102, 104 may keep their radio on all the time, primarily in the receiving mode.
A wireless mesh network 100 may be organized in clusters 110. This means that at least one node 102, called as a cluster head 102, in the cluster 1101 has taken responsibility for maintaining the time synchronization and frequency channel co-ordination among the cluster members 104. At least the head node 102 may take a role of routing node to forward data of the cluster members 104, while the rest of the nodes 104 may not do routing. The transmission powers are adjusted to operate within the cluster 110. Each cluster head node 102 needs to be a member of at least one other cluster 110 to be able to route data forward. In addition, typically only the cluster head 102 transmits data out of the cluster 110, to the next cluster 110 on the way towards the sink node(s) 106.